Fluorescence studies on the interaction of choline-binding domain B of the major bovine seminal plasma protein, PDC-109 with phospholipid membranes

The microenvironment and accessibility of the tryptophan residues in domain B of PDC-109 (PDC-109/B) in the native state and upon ligand binding have been investigated by fluorescence quenching, time-resolved fluorescence and red-edge excitation shift (REES) studies. The increase in the intrinsic fluorescence emission intensity of PDC-109/B upon binding to lysophosphatidylcholine (Lyso-PC) micelles and dimyristoylphosphatidylcholine (DMPC) membranes was considerably less as compared to that observed with the whole PDC-109 protein. The degree of quenching achieved by different quenchers with PDC-109/B bound to Lyso-PC and DMPC membranes was significantly higher as compared to the full PDC-109 protein, indicating that membrane binding afforded considerably lesser protection to the tryptophan residues of domain B as compared to those in the full PDC-109 protein. Finally, changes in red-edge excitation shift (REES) seen with PDC-109/B upon binding to DMPC membranes and Lyso-PC micelles were smaller that the corresponding changes in the REES values observed for the full PDC-109. These results, taken together suggest that intact PDC-109 penetrates deeper into the hydrophobic parts of the membrane as compared to domain B alone, which could be the reason for the inability of PDC-109/B to induce cholesterol efflux, despite its ability to recognize choline phospholipids at the membrane surface.